An absorbent article such as a disposable diaper generally includes: a front waistline portion to be fitted to the front waist of a wearer; a back waistline portion to be fitted to the back waist of the wearer; a crotch portion to be fitted to the crotch of the wearer; and leg-surrounding opening portions which open at both sides of the crotch portion. The front waistline portion and the back waistline portion are joined to each other at waist lateral-side portions.
In the steps of manufacturing a disposable diaper, a web is folded into two at the crotch portion by overlapping its first web to form the front waistline portion and its second web to form the back waistline portion with each other, and the web thus folded is joined at the left and right waist lateral-side portions, thereby forming the web into pants-type diapers. Here, ultrasonic joining using an ultrasonic joining apparatus is employed for the joining of the web at the left and right waist lateral-side portions (for example, Patent Document 1).
The ultrasonic joining apparatus includes: a rotating drum having an anvil formed thereon, the anvil being configured to form a joint pattern by which the front waistline portion and the back waistline portion are joined at the left and right waist lateral-side portions; and an ultrasonic horn configured to generate ultrasonic waves. The ultrasonic joining apparatus joins the first and second webs of the web, which is folded into two at the crotch portion, by pressing the ultrasonic horn intermittently against certain positions of the left and right waist lateral-side portions of the first and second webs while conveying the first and second webs between the rotating drum and the ultrasonic horn in a machine direction in parallel with a conveyance direction.
The anvil includes: a base being provided on the outer peripheral surface of the rotating drum; a projection being provided on the base and protruding in a normal direction of the rotating drum; and multiple protrusions protruding from the projection in the normal direction. When the ultrasonic horn is pressed against the anvil with the web interposed therebetween, the web is melted by being heated from the inside thereof, and is pressed against the protrusions protruding from the projection. As a result, a convexo-concave joint pattern is formed on the web.
An output value of the ultrasonic horn is adjusted to be suitable to the type and thickness of the web. Moreover, a clearance between each of the multiple protrusions and the ultrasonic horn is set to be small enough to apply ultrasonic vibration on the web, and large enough not to cut the web. Further, the anvil of the conventional ultrasonic joining apparatus is provided with a heater and a temperature sensor, thereby maintaining a constant temperature condition in the ultrasonic joining. With the above manner, the ultrasonic joining can be performed appropriately on the web.
In an ultrasonic joining apparatus disclosed in Patent Literature 1 in which multiple anvils are provided on the outer peripheral surface of one rotating drum, the peripheral length of the metal-made rotating drum slightly changes and the radial length thereof also changes if the temperature of the apparatus changes between phases where the ultrasonic joining apparatus starts its operation and where the apparatus continuously runs. In addition, all the anvils provided on the rotating drum never wear down at the same condition, and thus the amount of wear attributable to the repetitive use is different from one anvil to another.
In consideration of the above factors, a clearance between each protrusion of the anvil and the ultrasonic horn is different from one anvil to another. In the conventional ultrasonic joining apparatus, the magnitude of ultrasonic vibration and the temperature condition can be set as one of conditions allowing ultrasonic joining of the web. However, in the conventional ultrasonic joining apparatus, the clearance adjustment needs to be made frequently in order to maintain stable ultrasonic joining quality.
However, as described above, a clearance between the tip of each protrusion of the anvil and the ultrasonic horn is different from one anvil to another, thus making the clearance adjustment work extremely troublesome. Further, the frequent suspension of a manufacturing line for the clearance adjustment is impractical. To cope with this, a large tolerance for the clearances has been employed.